1. Field of the Invention
The present invention relates to processes for the manufacture of intensifying screens, particularly intensifying phosphor screens for use in radiographic imaging.
2. Background of the Art
There are at least two critical objectives in the production of radiographic images, particularly in medical radiographic images. One desired aspect of radiographic imaging is the faithfulness of the generated image to the object through which radiation was passed during imaging. Another important aspect, particularly during medical radiographic imaging, is the reduction of the level of exposure of the object (patient) to radiation during the imaging process.
One significant means of reducing the level of X-ray exposure during imaging has been the use of "intensifying screens" during the imaging process. These screens usually comprise phosphors in a binder on a carrier layer. The phosphors absorb X-ray radiation at a higher efficiency than does silver halide which is normally used in the hard-copy output of radiographic images. The phosphors not only absorb X-rays at an efficient rate, but can also phosphoresce (or fluoresce), emitting radiation at a wavelength other than the wavelength of X-rays which the phosphor absorbed. Depending upon the chemical nature and properties of the phosphor, the emitted radiation may be at essentially any wavelength between and including the infrared and ultraviolet wavelengths of the electromagnetic spectrum. Silver halide naturally absorbs radiation in the ultraviolet and near blue wavelengths, and can be spectrally sensitized to efficiently absorb radiation in other portions of the visible and the infrared spectrum. By using an X-ray absorbing phosphor intensifying screen and a silver halide emulsion spectrally sensitized to the wavelength of emission of the phosphor screen, the efficiency of an X-ray imaging system can be greatly enhanced. This allows for the use of lower doses of X-rays during exposure of the object.
The use of such phosphors is well known in the art as exemplified by U.S. Pat. Nos. 3,883,747 and 4,204,125, where there is direct emission of phosphorescent radiation upon X-ray stimulation. Other examples of such phosphors may be found in U.S. Pat. Nos. 3,859,527 and 5,164,224, where there is exposure to X-rays, storage of the absorbed energy by the phosphor, and subsequent stimulation by stimulating radiation to cause the phosphor to emit the stored energy as UV to infrared radiation. These phosphor systems are commercially successful and provide a significant benefit to the radiographic art. In these types of systems, however, there is a trade-off between speed and sharpness. The screen can be made thicker in order to absorb more X-rays and emit more light. Unfortunately, light generated within the screen is scattered by the phosphor grains to a greater extent in thicker screens, thereby reducing the resulting image sharpness recorded on the film. Conversely, to improve sharpness, a thinner screen is desirable, but a thinner screen requires a higher dosage to the patient or object being X-rayed due to reduced X-ray absorbing capability.
Many methods of improving the image quality, particularly the sharpness of images generated from phosphor screens without adversely affecting the sensitivity or speed of the system, have been proposed. Reflective particulates, dyes, pigments and other light affecting materials have been proposed as additives to phosphor layers to improve sharpness as shown in EPO 102 790 (powdered glass), Japanese Application 146,447/1980 (white pigments), Japanese Patent Application 163,500/1980 (colorants), and EPO 175 578 (sputtering or vacuum evaporation of phosphors). The objective of these methods is primarily to provide a high concentration of phosphor in the active layer of the screen and provide a screen of uniform properties. U.S. Pat. No. 5,306,367 produces a storage phosphor screen by dispersing phosphor particles in a thermoplastic binder diluted with a solvent, then coats the mixture, dries to remove the solvent, and compresses the coating at a temperature above the melting point of the binder.
U.S. Pat. No. 5,296,117 deposits phosphor particles in a binder by electrophoretic deposition of a dispersion of the phosphor particles in a solution of polymeric binder. The solution is coated onto a substrate, dried and the phosphor screen thus produced. Each of these types of systems has shown some benefits, but there is still significant room for improvement in the sharpness of radiographic phosphor screens. In particular, elimination of complicated or costly deposition processes, elimination of the use of environmentally harmful solvents, and reduction or elimination of high processing temperatures would be desirable.
Many radiation curable polymer systems are known. However, many of these systems do not provide the properties required of a binder for phosphor particles in intensifying screens. U.S. Pat. No. 5,091,483 discloses radiation curable organopolysiloxanes which may cure to an elastomer or be used in a pressure sensitive adhesive.